DISTINGUISHING TOOTH MORPHOTYPES FROM A MICROVERTEBRATE ASSEMBLAGE FROM THE CHINLE FORMATION (LATE TRIASSIC, ARIZONA) HELPS CHRONICLE ECOLOGICAL DIFFERENTIATION OF ARCHOSAUROMORPHS

Microvertebrate assemblages can provide substantial insight into the paleoecology and evolution of Mesozoic reptiles, but taxonomic and ecomorph identifications using qualitative characteristics remains difficult to reproduce. For this project, we quantified morphological differences in a large sample (n > 200) of fossil teeth from a Norian microvertebrate assemblage from the ‘Green Site’ locality (Chinle Formation, Arizona) to devise a replicable set of morphological characters to differentiate teeth. Comparing these data to Middle Triassic reptilian assemblages from similar paleoenvironments allows us to track trends in morphological diversity through the early archosauromorph radiation.

The Green Site, a rich deposit of predominantly fluvial, lacustrine, and floodplain sediments, lies within the Chinle Formation. The mud/sandstone matrix of the Green Layer contains abundant reptile, temnospondyl, chondrichthyan, and actinopterygian material. Biostratigraphy and constraints from uranium-lead dating of detrital zircons place the deposition of the Green Site locality approximately 216 Ma in the early Revueltian holochronozone.

Sediment was bulk-sampled from a freshly exposed outcrop, and all teeth were picked individually, after screen washing through a series of different sized sieves. Picked teeth were divided into dozens of morphotypes, delineated by shape rather than taxonomy. Morphotypes were then quantified using a combination of 14 discrete and three continuous variables to evaluate differences in shape and other features (e.g., serration shape and density).

Based on preliminary results, these variables allow us to differentiate morphotypes quantifiably and repeatably. Morphotype data were combined using non-metric multidimensional scaling. Clusters of indeterminate archosauromorphs may provide insight into several currently undifferentiated taxa within these morphotypes. An in-line/offset mesial carina also factors heavily into the differentiation of these morphotypes, which may prove important in future studies of Late Triassic radiations. The results, when compared with other microvertebrate and macrovertebrate assemblages, produce a more comprehensive picture of tooth differentiation during the Triassic archosauromorph radiation.